1. Field of the Invention
The present invention relates to a camera system having a focus detection device and, more particularly, to a lens-interchangeable digital single-lens reflex camera system having the focus detection device.
2. Related Background Art
FIG. 2 is a block diagram showing the main part of focus detection operation in a conventional lens-interchangeable silver halide film camera. Referring to FIG. 2, a lens body 1 includes an image taking optical system. The lens body 1 incorporates an image taking optical system 2 that is formed by one or a plurality of lens groups and can change the focal length by moving all or some of the lens groups, a lens state detection means 37 for detecting the focal length, i.e., the zoom state, of the image taking optical system 2, a driving means 3 for adjusting the focus state of the image taking lens 1 by moving all or some of the lenses forming the image taking optical system 2, a storage means 4 such as a ROM, and a lens control means 5 for controlling these components. In this case, the lens state detection means 37 detects the movement state of a lens or an amount that characterizes the movement state of the lens that moves to change the focal length (zoom state) of the image taking optical system 2 by a known method, e.g., using an electrode for an encoder which is provided for a lens barrel that rotates or moves to change the focal length of the image taking optical system 2, an electrode for detection which is in contact with the electrode for the encoder, and the like.
A camera (camera body) 6 incorporates a main mirror 7, a focusing screen 8 on which an object image is formed, a pentaprism 9 for image reversal, and an eyepiece lens 10. These elements constitute a finder system. This camera also includes a sub-mirror 11, a focus detection means 12, a computation means 13, a camera control means 14, and a film 15 serving as an image taking medium. The lens 1 and camera body 6 have contacts 16. While the lens 1 and camera body 6 are attached to each other, communication of various information and supply of power are performed through the contacts 16.
FIG. 13 is a view showing the focus detection means 12 having a plurality of focus detection points.
Referring to FIG. 13, a field mask 116 has a crucial aperture portion 116-1 in the center and vertically elongated aperture portions 116-2 and 116-3 in peripheral portions on two sides of the aperture portion 116-1. A field lens 117 is made up of three portions 117-1, 117-2, and 117-3 in correspondence with the three apertures 116-1, 116-2, and 116-3 of the field mask. A stop (aperture) 118 has an aperture portion 118-1 in the central portion. The aperture portion 118-1 has a total of four apertures 118-1a, 118-1b, 118-1c, and 118-1d located at upper, lower, left, and right positions each. In addition, two aperture portions 118-2 and 118-3 are respectively formed in the left and right peripheral portions of the stop 118. The areas 117-1, 117-2, and 117-3 of the field lens 117 respectively have the effects of imaging these aperture portions 118-1, 118-2, and 118-3 on portions near the exit pupil of an image taking optical system (not shown). An optical member 119 is an integral secondary imaging system made up of four pairs of lenses 119-1a and 119-1b, 119-c and 119-1d, 119-2a and 119-2b, and 119-3a and 119-3b, a total of eight lenses. These lenses are arranged behind the respective apertures of the stop 118. A photoelectric conversion element 120 is constituted by four pairs of sensor arrays 120-1a and 120-1b, 120-1c and 120-1d, 120-2a and 120-2b, and 120-3a and 120-3b, a total of eight sensor arrays. These sensor arrays are arranged in correspondence with the respective lenses of the secondary imaging system to receive image light beams from the lenses.
FIG. 14 shows how an object image is formed on the photoelectric conversion element 120. After light beams transmitted through the central aperture portion 116-1 of the field mask 116 and the central portion 117-1 of the field lens 117 are partially selected by the apertures 118-1a, 118-1b, 118-1c, and 118-1d of the stop, image areas 121-1a, 121-1b, 121-1c, and 121-1d are formed on the photoelectric conversion element 120 by the lenses 119-1a, 119-1b, 119-c, and 119-d of the secondary imaging system 119 located behind the apertures. After light beams transmitted through the peripheral aperture portion 116-3 of the field mask 116 and the peripheral portion 117-3 of the field lens 117 are partially selected by the apertures 118-3a and 118-3b of the stop 118, image areas 121-2a and 121-2b are formed on the photoelectric conversion element 120 by the lenses 119-3a and 119-3b of the secondary imaging system 119 located behind the apertures. The focus detection principle of the focus detection means shown in FIG. 13 is generally called a phase difference detection scheme. When the imaging point of the image taking optical system 2 is located in front of an expected focal plane, i.e., on the image taking optical system 2 side, light amount distributions associated with object images formed on a pair of sensor arrays come close to each other. In contrast to this, when the imaging point of the image taking optical system 2 is located behind the expected focal plane, i.e., on the opposite side to the image taking optical system 2, light amount distributions associated with object images formed on the pair of sensor arrays separate from each other. The offset amount between the light amount distributions associated with object images formed on a pair of sensor arrays has a function relationship with the defocus amount of the image taking optical system 2, i.e., the focus deviation amount. If, therefore, the offset amount is calculated by a proper computation means, the direction and amount of focus deviation of the image taking optical system 2 can be detected. Assume that these focus detection means are used for a camera capable of interchanging image taking lenses such as a single-lens reflex camera. In this case, however, if a lens is controlled on the basis of a focus state detection signal associated with a focus deviation amount which is directly obtained from the focus detection means, a proper focus state may not be obtained. The main reason for this is that an image taking optical system for forming an image to be observed or taken and a focus detection means generally receive different light beams. In addition, the focus detection means based on the phase difference detection scheme obtains a focal position or focus deviation amount to be determined on the basis of the amount of aberration in the vertical (optical axis) direction upon converting it into an image deviation associated with aberration in the horizontal direction. For this reason, when aberration occurs in the image taking optical system, the two values may differ from each other depending on the aberration correction state.
To solve this problem, a correction means is provided to correct a focus detection signal D representing a focus deviation amount by
DC=Dxe2x88x92Cxe2x80x83xe2x80x83(1)
using a unique correction value C for each image taking lens, and the driving means 3 is used to drive the image taking optical system entirely or partly on the basis of an obtained correction focus detection signal DC, thereby controlling the lens to match the best imaging position with respect to the film surface. In this case, the best imaging position is the peak position of an MTF corresponding to an on-axis spatial frequency of 30 lines/mm.
FIG. 3 shows a conventional lens-interchangeable digital single-lens reflex camera. Most conventional lens-interchangeable digital single-lens reflex cameras use the bodies of silver halide single-lens reflex cameras, and hence the arrangement in FIG. 3 is almost the same as that in FIG. 2. In a digital single-lens reflex camera, however, an image taking element such as a CCD is used as an image taking plane 15 in place of a film. In addition, an optical member 40 such as a low-pass filter is interposed between the image taking optical system and the image taking element to prevent moire caused by sampling in the image taking element. Focus detection operation in such a lens-interchangeable digital single-lens reflex camera is the same as the above focus detection operation.
If, however, such focus detection operation is performed in a digital single-lens reflex camera, since a light beam guided to the focus detection device does not pass through optical members such as the low-pass filter and the cover glass of the image taking element, this light beam differs from a light beam that is guided to the image taking element and passes through the low-pass filter and the cover glass of the image taking element. For this reason, the best imaging position detected by the focus detection device deviates from the best imaging position on the image taking plane side.
FIG. 4 shows this state, i.e., the imaged state on the optical axis. FIG. 4 shows an image taking optical system 141 and an optical member 142 such as a low-pass filter. If the optical member is not used, focus detection operation is performed by the above focus detection device, and a lens is controlled to match a best imaging position 144 to an image taking plane 143 by an operation for correcting them. However, since the optical member 142 such as a low-pass filter is placed between the image taking optical system and the image taking element, a ray of light that should form an image on the image taking plane 143 is refracted at an incident or emergence surface of the optical member 142, and the best imaging position 144 changes to a best imaging position 145. In a digital single-lens reflex camera using the body of a conventional silver halide single-lens reflex camera, however, a change in the best imaging position due to the optical member cannot be detected and corrected in focus detection operation. If a lens is controlled on the basis of the information of the best imaging position detected by the focus detection device, the image taking plane does not coincide with the best imaging position.
The problem in the prior art described above is associated with an error in focus detection due to a change in the best imaging position of a light beam which forms an on-axis image. The same problem arises when focus detection is performed for a light beam which forms an off-axis image.
The present invention has been made in consideration of the above situation, and has as its object to provide a focus detection device capable of obtaining a proper focus detection state in a digital single-lens reflex camera having a focus detection device having a single or a plurality of focus detection points, and a camera system using the focus detection device.
In order to solve the above problem, according to the present invention, a lens-interchangeable single-lens reflex camera system includes a focus detection means for obtaining a signal associated with the focus state of the image taking optical system with respect to a predetermined area on the expected focal plane of the image taking optical system, and an optical member (low-pass filter) located between an image taking element and the image taking optical system, and a camera unit has a storage means for storing correction data for correcting the difference between the best imaging position and the detection result obtained by the focus detection means which is caused by the optical member, so that a change in best imaging position at each focus detection point is corrected on the basis of the correction data stored in the storage means.
Other objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings.